Systems and methods for suppressing and mitigating harmonic distortion in a circuit

ABSTRACT

Systems and methods for suppressing and mitigating harmonic distortion in a circuit are disclosed. In one example, a disclosed circuit includes a radio frequency (RF) oscillator and a power amplifier. The RF oscillator is configured to generate an RF signal. The power amplifier is configured to generate an amplified RF signal based on the RF signal. The power amplifier includes a transformer including a primary winding and a secondary winding, and a feedback capacitor electrically coupled to the primary winding and the secondary winding.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional PatentApplication No. 62/643,583, filed on Mar. 15, 2018, which isincorporated by reference herein in its entirety.

BACKGROUND

Ultra-low power (ULP) radios underpin short-range communications forwireless Internet of Things (IoT). Yet, the lifetime of an IoT systemstill tends to be severely limited by a transmitter power consumptionand available battery technology. A radio frequency (RF) transmitter,e.g. a ULP transmitter, usually includes a phase locked loop (PLL) thatgenerates an output signal associated with a phase related to a phase ofan input signal.

A PLL, e.g. an all-digital PLL, may employ a digitally controlledoscillator (DCO) with a digital power amplifier (DPA) for switchingcurrent sources to reduce supply voltage and power without sacrificingits startup margin. When the DPA has a harmonic distortion at afrequency similar to the operating frequency of the DCO, injectionpulling occurs at the DCO to pull its operating frequency with theharmonic distortion. As such, the efficiency of an RF transmitterincluding a DCO and a DPA will be hurt by the harmonic distortion aswell as the injection pulling.

Thus, conventional transmitter circuits are not entirely satisfactory.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that various features are not necessarily drawn to scale. In fact,the dimensions and geometries of the various features may be arbitrarilyincreased or reduced for clarity of discussion. Like reference numeralsdenote like features throughout specification and drawings.

FIG. 1 illustrates a portion of an exemplary radio frequency (RF)circuit including a digitally controlled oscillator (DCO) and a digitalpower amplifier (DPA), in accordance with some embodiments of thepresent disclosure.

FIG. 2 illustrates an interaction between a DCO and a DPA of anexemplary RF circuit, in accordance with some embodiments of the presentdisclosure.

FIG. 3A illustrates a feedback capacitor in a single-ended DPA of anexemplary RF circuit, in accordance with some embodiments of the presentdisclosure.

FIG. 3B illustrates functions performed by a single-ended DPA having afeedback capacitor in an exemplary RF circuit, in accordance with someembodiments of the present disclosure.

FIG. 3C illustrates performances of a single-ended DPA having a feedbackcapacitor in an exemplary RF circuit, in accordance with someembodiments of the present disclosure.

FIG. 4A illustrates a mitigation of injection pulling from a DPA to aDCO in an exemplary RF circuit, in accordance with some embodiments ofthe present disclosure.

FIG. 4B illustrates performances of the injection pulling mitigationshown in FIG. 4A, in accordance with some embodiments of the presentdisclosure.

FIG. 5 illustrates a concentric octagon layout topology of an exemplaryRF circuit including a DCO and a DPA, in accordance with someembodiments of the present disclosure.

FIG. 6 illustrates an exemplary method for suppressing a harmonicdistortion emitted by a power amplifier, in accordance with someembodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure describes various exemplary embodiments forimplementing different features of the subject matter. Specific examplesof components and arrangements are described below to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting. For example, the formation of a first featureover or on a second feature in the description that follows may includeembodiments in which the first and second features are formed in directcontact, and may also include embodiments in which additional featuresmay be formed between the first and second features, such that the firstand second features may not be in direct contact. In addition, thepresent disclosure may repeat reference numerals and/or letters in thevarious examples. This repetition is for the purpose of simplicity andclarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Digitally controlled oscillator (DCO) and digital power amplifier (DPA)are common components in a radio frequency (RF) circuit. When bothcomponents are coupled in the circuit, the DPA may emit a harmonic at afrequency similar to the operating frequency of the DCO, which causesinjection pulling or even injection locking at the DCO.

The present disclosure aims at reducing or removing the injectionpulling at the oscillator to enable the circuit to operate in low supplyvoltage and easily overcome process variation. In one embodiment, adisclosed circuit includes a power amplifier having a capacitor and amatching network transformer operating in an inverting configuration.The capacitor is electrically coupled between a primary winding and asecondary winding of the matching network transformer. Serving as afeedback capacitor, the capacitor can enhance a feedback coupling fromthe secondary winding to the primary winding at a harmonic, to suppressor cancel an emission of harmonic distortion at this harmonic, therebymitigating an injection pulling at an oscillator in the circuit.

In another embodiment, the oscillator comprises a coil magneticallycoupled between the primary winding and the secondary winding of thepower amplifier to sense the coupling and to send an amplified butinverted signal back to the windings to cancel or compensate theharmonic injection into the oscillator. The compensating strength of thecoil may be controlled by a bias voltage to make the magnetic couplingfactor between the power amplifier, and the oscillator at the harmonicbelow a certain threshold. For example, the harmonic may be asecond-order harmonic or a third-order harmonic.

The present disclosure is applicable to all kinds of circuits includinga phase locked loop and/or a frequency synthesizer. The disclosedsolution can save circuit area with an easy implementation and portingscheme.

FIG. 1 illustrates a portion of an exemplary radio frequency (RF)circuit 100 including a digitally controlled oscillator (DCO) 110 and adigital power amplifier (DPA) 120, in accordance with some embodimentsof the present disclosure. As shown in FIG. 1, the exemplary RF circuit100 includes a serial peripheral interface (SPI) 102, the DCO 110, afrequency divider 112, and the DPA 120.

The DCO 110 in this example may receive an oscillator tuning word OTW104 through the SPI 102, e.g. from a control word generator (not shownin FIG. 1). According to the oscillator tuning word OTW 104, the DCO 110may generate an oscillator signal, e.g. a radio frequency (RF) signalfor transmission. In some embodiments, the oscillator tuning word OTW104 provides a corresponding operating voltage to control the frequencyof the oscillator signal. Thus, the DCO 110 is configured to vary afrequency of the oscillator signal based upon the oscillator tuning wordOTW. For a PLL, over a plurality of clock cycles, the oscillator tuningword OTW 104 drives the circuit 100 to enter a locked state byminimizing a phase error signal. The output oscillator signal in thisexample has a frequency of 2f₀.

In some embodiments, the frequency divider 112 is configured to dividethe frequency of the oscillator signal to output a down-dividedoscillator signal. In this example, the frequency divider 112 dividesthe frequency 2f₀ of the oscillator signal by two to output thedown-divided oscillator signal with a frequency of f₀. For example, whenthe frequency of the oscillator signal is around 1.8 GHz, the frequencyof the down-divided oscillator signal is around 0.9 GHz. An inputterminal of the frequency divider 112 is electrically coupled to theoutput terminal of the DCO 110 to receive the oscillator signal. Anoutput terminal of the frequency divider 112 is electrically coupled tothe DPA 120. The down-divided oscillator signal may serve as an RF clocksignal for the DPA 120.

The DPA 120 in this example is configured to generate an output signalat an output node 128 according to the down-divided oscillator signal.The DPA 120 in this example includes an op-amp comparator 121, athermometer encoder 123, and a power amplifier (PA) array 126, e.g. aclass-E PA array. The op-amp comparator 121 may compare two inputvoltage signals to generate a low-voltage differential signaling (LVDS)output as an amplitude control word ACW 122. The op-amp comparator 121outputs the amplitude control word ACW 122 to the thermometer encoder123 for encoding. In one embodiment, the DPA 120 may be a single-endeddifferential DPA.

The thermometer encoder 123 in this example receives and encodes theamplitude control word ACW 122 from the op-amp comparator 121. Thethermometer encoder 123 serves as a modulator to generate digitalsignals to control the PA array 126 to adjust the amplitude of theoutput signal at the output node 128. The op-amp comparator 121 and thethermometer encoder 123 form an analog-to-digital convertor (ADC). Thethermometer encoder 123 generates an encoded ACW at different tuningstrengths. The encoded ACW may be used to coarse-tune and fine-tune theamplitude of the output signal of the PA array 126. The thermometerencoder 123 may send both the coarse-tuning signal and the fine-tuningsignal to the PA array 126 for power amplification.

The PA array 126 in this example receives the coarse-tuning signal andthe fine-tuning signal from the thermometer encoder 123 and receives theRF clock signal of frequency f₀ from the frequency divider 112. Theoutput signal of the PA array 126, i.e. the output signal of the DPA 120at the output node 128, may be an amplified signal of the down-dividedoscillator signal and have a same fundamental frequency f₀ as that ofthe down-divided oscillator signal or the RF clock signal. But when apower amplifier amplifies a signal, in addition to the fundamentalfrequency, harmonics (e.g. second-order harmonic 2f₀ or third-orderharmonic 3f₀) may also be amplified, which can cause harmonic distortion(HD) emission from the DPA 120 to the DCO 110 by an injection pullingdue to a magnetic coupling between the DCO 110 and the DPA 120. Forexample, if the DPA 120 emits a second-order harmonic distortion (HD2)at the frequency 2f₀ from a matching network transformer inside the DPA120, an injection pulling may occur at the DCO 110 whose internaltransformer also works around the frequency 2f₀. To avoid this injectionpulling, a pulling mitigation mechanism is added in the exemplary RFcircuit 100, with respect to the HD2 at the frequency 2f₀. In variousembodiments, the pulling mitigation mechanism may work for a circuitincluding any oscillator and any power amplifier. In variousembodiments, the pulling mitigation mechanism may mitigate injectionpulling for harmonic distortions other than HD2 as well.

As an example of the pulling mitigation mechanism, the PA array 126 maycomprise a transformer and a feedback capacitor. The transformerincludes a primary winding and a secondary winding. The feedbackcapacitor is electrically coupled to the primary winding and thesecondary winding, to enhance a feedback coupling from the secondarywinding to the primary winding to suppress at least one harmonicdistortion, e.g. a second-order harmonic distortion (HD2), of theamplified RF signal generated by the PA array 126.

FIG. 2 illustrates an interaction between a DCO 210 and a DPA 220 of anexemplary RF circuit 200, in accordance with some embodiments of thepresent disclosure. In one embodiment, the DCO 210 and the DPA 220 arecoupled to each other as the DCO 110 and the DPA 120 in FIG. 1. As shownin FIG. 2, the DCO 210 in this example includes a transformer having aprimary winding 211 and a secondary winding 212; and the DPA 220 in thisexample includes a transformer having a primary winding 221 and asecondary winding 222. Because the two transformers are located close toeach other in the exemplary RF circuit 200, they can be coupled to eachother magnetically, which may cause an injection pulling at the DCO 210.For example, the DCO 210 operates at the frequency 2f₀, and sends an RFclock signal at the frequency f₀ through a divider to the DPA 220 foramplification. After the DPA 220 amplifies the RF clock signal, theamplified signal may have harmonics, e.g. the second-order harmonic atthe frequency 2f₀, the third-order harmonic at the frequency 3f₀, etc.,in addition to the fundamental frequency f₀. Because the DPA 220 islocated close to the DCO 210 and because the second-order harmonic 2f₀is same as the operating frequency of the DCO 210, the component of theamplified signal at the frequency 2f₀ can cause injection pulling at theDCO 210 through the magnetic coupling between the two transformers ofthe DCO 210 and the DPA 220.

As shown in FIG. 2, one possible pulling mitigation mechanism is basedon a feedback capacitor 226 that is electrically coupled to the primarywinding 221 and the secondary winding 222 of the matching networktransformer in the DPA 220. In one embodiment, the feedback capacitor226 enhances a feedback coupling from the secondary winding 222 to theprimary winding 221 of the matching network transformer at thesecond-order harmonic 2f₀ to cancel or at least suppress the HD2emission from the DPA 220. In one embodiment, the matching networkincluding the primary winding 221 and the secondary winding 222 has aloaded quality factor (Q-factor) that is high enough such that athird-order harmonic distortion (HD3) emission is also suppressed belowa given threshold due to a filtering function of the matching network.

As shown in FIG. 2, another possible pulling mitigation mechanism isbased on a coil or winding 218 that is magnetically coupled between theprimary winding 221 and the secondary winding 222 of the matchingnetwork transformer in the DPA 220. The coil 218 is electrically coupledto the DCO 210 and is controlled by tunable bias 215, 216 associatedwith the coil 218. A bias voltage of the tunable bias 215, 216 may becontrolled based on the coil 218's sensing of the coupling between theprimary winding 221 and the secondary winding 222. By controlling thebias voltage, the DCO 210 can send an amplified but inverted signal backto the primary winding 221 and the secondary winding 222, through thecoil 218, to cancel or suppress the harmonic distortion from the DPA220. In addition, by controlling the bias voltage based on the sensedinformation at the coil 218, a coupling factor between the DCO 210 andthe DPA 220 becomes small, i.e. the injection pulling from the DPA 220to the DCO 210 is mitigated.

FIG. 3A illustrates a feedback capacitor 326 in a single-ended DPA 300of an exemplary RF circuit, in accordance with some embodiments of thepresent disclosure. In an embodiment shown in FIG. 3A, the RF circuitutilizes the feedback capacitor 326 to cancel or suppress harmonicdistortion from the single-ended DPA 300. In this example, thesingle-ended DPA 300 includes a PA array circuit 320 that has a singleend output 328.

The thermometer encoder 313 in this example generates an encodedamplitude control word ACW and sends to the PA array circuit 320. The PAarray circuit 320 receives the encoded amplitude control word ACW fromthe thermometer encoder 313, and receives an RF clock signal having afrequency of a 0.9 GHz from e.g. a DCO. Based on different tuningstrengths, the PA array circuit 320 can amplify the RF clock signal togenerate an amplified RF signal at the single end output 328. Thefeedback capacitor 326 in the PA array circuit 320 is electricallycoupled between a primary winding 321 and a secondary winding 322 of thePA array circuit 320. As discussed above, the feedback capacitor 326 inthe PA array circuit 320 can help to enhance the coupling factor kmbetween the primary winding 321 and the secondary winding 322, to cancelor suppress a harmonic distortion emitted by the single-ended DPA 300. Adetailed description of the simple feedback coupling cancellation (FBCC)is provided below with respect to FIG. 3B.

FIG. 3B illustrates functions performed by a single-ended DPA, e.g. thesingle-ended DPA 300 in FIG. 3A, having a feedback capacitor 326 in anexemplary RF circuit, in accordance with some embodiments of the presentdisclosure. In an example as shown in FIG. 3B, the feedback capacitor326 receives an HD2 signal represented by a sinusoidal wave from theside of the primary winding 321 and generates an inverted HD2 signalrepresented by an inverted sinusoidal wave at the side of the secondarywinding 322. The inverted HD2 signal is sent back as a feedback to theside of the primary winding 321, via the coupling between the primarywinding 321 and the secondary winding 322, to compensate or cancel theoriginal HD2 signal. In an ideal case, the compensated HD2 signal willhave no amplitude component at the HD2 frequency, i.e. totalcancellation of HD2 at the single end output 328.

FIG. 3C illustrates performances of a single-ended DPA, e.g. thesingle-ended DPA 300 in FIG. 3A, having a feedback capacitor, e.g. thefeedback capacitor 326 in FIG. 3A and FIG. 3B, in an exemplary RFcircuit, in accordance with some embodiments of the present disclosure.As shown in FIG. 3C, the output power 382 at the single end output 328of the single-ended DPA 300 is more than 11 dBm at the fundamentalfrequency 900 MHz. The PA efficiency 384 of the single-ended DPA 300 ishighest at the fundamental frequency 900 MHz. In addition, the HD2 power386 is lowest when the fundamental frequency is at 900 MHz.Specifically, the harmonic distortion emission performance 388 is shownwith a bias voltage equal to 0, i.e. no compensating path with extracoil other than the feedback capacitor 326 or compensating path turnedoff. With a fundamental frequency 900 MHz, the HD2 emission at frequency1.8 GHz is suppressed to −50 dBc, and the HD3 emission at frequency 2.7GHz is suppressed to −47 dBc, without any calibration.

FIG. 4A illustrates a mitigation of injection pulling from a DPA to aDCO in an exemplary RF circuit 400, in accordance with some embodimentsof the present disclosure. As shown in FIG. 4A, the exemplary RF circuit400 includes a DPA 420, a divider 422, and a DCO. The DCO in FIG. 4Acovers all components other than those of the divider 422 and the DPA420. The DCO includes a DCO transformer having a primary winding 411 anda secondary winding 412 that are magnetically coupled to each other. Inthis example, the primary winding 411 is coupled to a power supplyvoltage VDD of 0.3 V; the secondary winding 412 is coupled to a biasvoltage V_(B).

The divider 422 is coupled between the DCO and the DPA 420. As discussedabove, after the DCO generates an oscillator signal, e.g. at a frequencyof 1.8 GHz, the divider 422 divides the frequency of the oscillatorsignal to generate a clock signal having an average frequencyrepresenting a frequency of the oscillator signal divided by two. TheDPA 420 is configured to generate an amplified RF signal having afundamental frequency 0.9 GHz based on the clock signal from the divider422.

The DPA 420 in this example includes a DPA transformer having a primarywinding 421 and a secondary winding 422 that are magnetically coupled toeach other. While the DPA transformer including the primary winding 421and the secondary winding 422 operates at the fundamental frequency 0.9GHz, its HD2 emission is at the frequency 1.8 GHz that is the same asthe operating frequency of the DCO transformer including the primarywinding 411 and the secondary winding 412.

To mitigate the injection pulling from the DPA transformer to the DCOtransformer at the HD2 frequency and/or other harmonic frequencies, theDCO also includes a compensating circuit 430 including or coupled to acoil 418. The coil 418 is magnetically coupled between the primarywinding 421 and the secondary winding 422 to sense the coupling of theDPA transformer. The compensating circuit 430 controls a bias voltageV_(T) 438 associated with the coil 418, e.g. based on the sensing resultof the coil 418, to mitigate a magnetic coupling with respect to atleast one harmonic injection from the DPA to the DCO, e.g. by sending anamplified by inverted signal as a magnetic coupling feedback from theDCO to the DPA. In one embodiment, the at least one harmonic injectioncomprises a second-order harmonic injection to the DCO.

In this example, the compensating circuit 430 further comprises: a firstpair of transistors 432, 434 whose gates are coupled to two ends of thecoil 418 respectively; and a second pair of transistors 431, 433 whosegates are coupled to two ends of the secondary winding 412 respectively.The transistor 431 and the transistor 432 are connected in series. Thetransistor 433 and the transistor 433 are connected in series. Without afeedback capacitor, the exemplary RF circuit 400 in this exampleutilizes the sensing coil 418 and the compensating circuit 430 tomitigate the DPA-induced injection pulling at the DCO, through amagnetic coupling feedback from the DCO to the DPA controlled by thebias voltage V_(T) 438. For example, by controlling the bias voltageV_(T) 438, the voltage and the inductance of the coil 418 may be tunedto impact the coupling factor between the DCO transformer (including theprimary winding 411 and the secondary winding 412) and the DPAtransformer (including the primary winding 421 and the secondary winding422), by sending a magnetic coupling feedback to the DPA transformerfrom the coil 418. The injection pulling is mitigated when the couplingfactor between the DCO transformer and the DPA transformer is controlledto be lower than a certain threshold.

FIG. 4B illustrates performances of the injection pulling mitigationshown in FIG. 4A, in accordance with some embodiments of the presentdisclosure. As shown in FIG. 4B, the injection pulling spur 482 inducedby the DPA at the DCO is mitigated to −57.5 dBc with a 5 MHz frequencyoffset and the bias voltage V_(T) 438 tuned to 0.3 V. In this example,for different values of the bias voltage V_(T) 438, the injectionpulling spur (i.e. the injection pulled-back signal) 484 induced by theDPA at the DCO is minimized at 0.3 V.

FIG. 5 illustrates a concentric octagon layout topology 500 of anexemplary RF circuit including a DCO and a DPA, in accordance with someembodiments of the present disclosure. In this exemplary concentricoctagon layout topology 500, the DPA transformer is placed within aninner diameter of the DCO transformer. The DPA transformer includes aprimary winding 521 and a secondary winding 522, according to an octagonlayout with a smaller size. The DCO transformer includes a primarywinding 511 and a secondary winding 512, according to an octagon layoutwith a larger size. The larger octagon layout is outside and concentricto the smaller octagon layout of the DPA transformer. The primarywinding 511 is coupled to a power supply voltage VDD 516. The secondarywinding 512 is coupled to a bias voltage V_(B) 514.

In this example, the DCO transformer is electrically coupled to a coil518. The coil 518 is placed between the primary winding 521 and thesecondary winding 522 of the DPA transformer in the smaller octagonlayout. The coil 518 is magnetically coupled to the primary winding 521and the secondary winding 522, and can sense the coupling of the DPAtransformer to send an amplified but inverted signal back to the DPAtransformer windings to cancel or mitigate the HD2 injected into theDCO.

In one embodiment, a minimum distance is reserved between an outmostturn of the DPA transformer and an innermost turn of the DCO transformerto keep a coupling factor at a certain harmonic between the DPA and theDCO below a given threshold.

FIG. 6 illustrates an exemplary method 600 for suppressing a harmonicdistortion emitted by a power amplifier having a transformer, inaccordance with some embodiments of the present disclosure. As shown inFIG. 6, a first signal having a harmonic distortion with respect to agiven order harmonic is generated at operation 602 at a primary windingof the transformer. The first signal is sent at operation 604 throughthe feedback capacitor to generate a second signal at a secondarywinding of the transformer. The given order harmonic of the secondsignal is an inversion of the given order harmonic of the first signal.The second signal is sent at operation 606 as a feedback to the primarywinding through a magnetic coupling between the primary winding and thesecondary winding. The first signal is compensated at operation 608 withthe second signal to suppress the harmonic distortion.

It can be understood that the order of the steps shown in FIG. 6 may bechanged according to different embodiments of the present disclosure.

In an embodiment, a circuit is disclosed. The circuit includes: a radiofrequency (RF) oscillator and a power amplifier. The RF oscillator isconfigured to generate an RF signal. The power amplifier is configuredto generate an amplified RF signal based on the RF signal. The poweramplifier includes a transformer including a primary winding and asecondary winding, and a feedback capacitor electrically coupled to theprimary winding and the secondary winding.

In another embodiment, a circuit is disclosed. The circuit includes: aradio frequency (RF) oscillator and a power amplifier. The RF oscillatoris configured to generate an RF signal. The power amplifier isconfigured to generate an amplified RF signal based on the RF signal.The power amplifier includes a first transformer including a firstprimary winding and a first secondary winding. The RF oscillatorcomprises a coil magnetically coupled between the first primary windingand the first secondary winding.

In yet another embodiment, a method for suppressing harmonic distortionsof a power amplifier comprising a transformer and a feedback capacitoris disclosed. The method includes: generating a first signal having aharmonic distortion with respect to a given order harmonic at a primarywinding of the transformer; sending the first signal through thefeedback capacitor to generate a second signal at a secondary winding ofthe transformer, wherein the given order harmonic of the second signalis an inversion of the given order harmonic of the first signal; sendingthe second signal as a feedback to the primary winding through amagnetic coupling between the primary winding and the secondary winding;and compensating the first signal with the second signal to suppress theharmonic distortion.

The foregoing outlines features of several embodiments so that thoseordinary skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A circuit, comprising: a radio frequency (RF)oscillator configured to generate an RF signal; and a power amplifierconfigured to generate an amplified RF signal based on the RF signal,wherein the power amplifier comprises: a transformer including a primarywinding and a secondary winding, and a feedback capacitor electricallycoupled to the primary winding and the secondary winding.
 2. The circuitof claim 1, wherein: the RF oscillator is a digitally controlledoscillator (DCO) that generates the RF signal according to an oscillatortuning word; and the power amplifier is a single-ended differentialdigital power amplifier (DPA).
 3. The circuit of claim 1, wherein thefeedback capacitor enhances a feedback coupling from the secondarywinding to the primary winding to suppress at least one harmonicdistortion of the amplified RF signal.
 4. The circuit of claim 3,wherein the at least one harmonic distortion comprises a second-orderharmonic distortion (HD2) of the amplified RF signal.
 5. The circuit ofclaim 4, further comprising a frequency divider that is coupled betweenthe RF oscillator and the power amplifier and configured to generate aclock signal having an average frequency representing a frequency of theRF signal divided by two, wherein the power amplifier is configured togenerate the amplified RF signal based on the clock signal.
 6. Thecircuit of claim 3, wherein the power amplifier comprises a matchingnetwork having a loaded quality factor that is high enough such that athird-order harmonic distortion (HD3) of the amplified RF signal issuppressed below a given threshold by the feedback capacitor.
 7. Thecircuit of claim 1, wherein the transformer of the power amplifier isplaced within an inner diameter of a transformer of the RF oscillatoraccording to a concentric layout topology.
 8. The circuit of claim 1,the RF oscillator comprises a coil magnetically coupled between theprimary winding and the secondary winding.
 9. A circuit, comprising: aradio frequency (RF) oscillator configured to generate an RF signal; anda power amplifier configured to generate an amplified RF signal based onthe RF signal, wherein: the power amplifier comprises a firsttransformer including a first primary winding and a first secondarywinding, and the RF oscillator comprises a coil magnetically coupledbetween the first primary winding and the first secondary winding. 10.The circuit of claim 9, wherein: the RF oscillator is a digitallycontrolled oscillator (DCO) that generates the RF signal according to anoscillator tuning word; and the power amplifier is a single-endeddifferential digital power amplifier (DPA).
 11. The circuit of claim 9,wherein the RF oscillator further comprises a compensating circuitcoupled to the coil and configured to control a bias voltage associatedwith the coil.
 12. The circuit of claim 11, wherein the bias voltage iscontrolled to mitigate a magnetic coupling with respect to at least oneharmonic injection from the power amplifier to the RF oscillator. 13.The circuit of claim 12, wherein the at least one harmonic injectioncomprises a second-order harmonic injection to the RF oscillator. 14.The circuit of claim 13, further comprising a frequency divider that iscoupled between the RF oscillator and the power amplifier and configuredto generate a clock signal having an average frequency representing afrequency of the RF signal divided by two, wherein the power amplifieris configured to generate the amplified RF signal based on the clocksignal.
 15. The circuit of claim 11, wherein the RF oscillator furthercomprises a second transformer including a second primary winding and asecond secondary winding.
 16. The circuit of claim 15, wherein thecompensating circuit comprises: a first pair of transistors whose gatesare coupled to two ends of the coil respectively; and a second pair oftransistors whose gates are coupled to two ends of the second secondarywinding respectively, wherein each of the second pair of transistors isconnected to a respective one of the first pair of transistors inseries.
 17. The circuit of claim 15, wherein: the first transformer ofthe power amplifier is placed within an inner diameter of the secondtransformer of the RF oscillator according to a concentric layouttopology; and a minimum distance is reserved between an outmost turn ofthe first transformer and an innermost turn of the second transformer tokeep a coupling factor at a certain harmonic between the power amplifierand the RF oscillator below a given threshold.
 18. A method forsuppressing harmonic distortions of a power amplifier comprising atransformer and a feedback capacitor, comprising: generating a firstsignal having a harmonic distortion with respect to a given orderharmonic at a primary winding of the transformer; sending the firstsignal through the feedback capacitor to generate a second signal at asecondary winding of the transformer, wherein the given order harmonicof the second signal is an inversion of the given order harmonic of thefirst signal; sending the second signal as a feedback to the primarywinding through a magnetic coupling between the primary winding and thesecondary winding; and compensating the first signal with the secondsignal to suppress the harmonic distortion.
 19. The method of claim 18,further comprising: sensing the magnetic coupling between the primarywinding and the secondary winding by a coil magnetically coupled betweenthe primary winding and the secondary winding; and sending, by the coil,an amplified and inverted signal to the primary winding and thesecondary winding, wherein the coil is electrically coupled to anoscillator.
 20. The method of claim 19, further comprising: controllinga bias voltage associated with the coil to mitigate a magnetic couplingwith respect to at least one harmonic injection from the power amplifierto the oscillator.